The present invention relates to optical fiber. More particularly, the present invention relates to metal-coated optical fiber, and techniques for manufacturing same.
Optical fiber is typically constructed having a polymer coating, but some applications necessitate the use of metal-coated optical fiber. One problem with metal-coated optical fiber is that it is not commercially available in long lengths.
Current metal-coated optical fibers are typically manufactured by a liquid freezing method or a metal plating method. The liquid freezing method is described in detail in A. Mendez & T. F. Morse, Specialty Optical Fibers Handbook, Academic Press (2007), at pages 491-510 (“Metal-Coated Fibers”), which is incorporated herein by reference in its entirety for all purposes. As a brief description of the liquid freezing method, optical fiber is coated with metal by the fiber passing though a die filled with liquid metal or molten metal in line with a fiber drawing process.
Particularly when the thickness of metal coating is decreased less than ten micron, this freezing process has the possibility of mechanical contact of optical fiber with the coating die due to small fluctuation of the drawing tower or environmental conditions (such as temperature change, wind, vibration, etc.). A bare fiber without a thick polymer coating is fragile against handling or mechanical contact with any hard material. (A thin carbon layer is sometimes coated in line with drawing as a hermetic barrier. It will be appreciated that an optical fiber with a carbon layer less than one micron without any additional coating is still fragile against normal handling) So any mechanical contact with hard material can cause mechanical damage on the optical fiber surface and can degrade long term mechanical reliability.
In order to avoid mechanical contact of fiber with the coating die, the typical coating thickness of metal-coated fiber made by a liquid freezing method is larger than ten micron to obtain enough mechanical strength. However, the transmission loss of fiber with a thicker metal coating is larger due to thick metal thermal contraction. In particular, the contraction of the metal layer causes microbending loss when metal shrinks from liquid phase to solid phase due to the thermal expansion coefficient.
For metal-coated fiber exhibiting lower losses, thicker glass diameter needs to be selected (e.g., more than about 200 micron) to resist microbending due to metal contraction. But thick diameter of 200 micron or greater limits bending radius due to larger bending strain.
Difficulties in manufacturing metal plated optical fiber are due, in part, to incompatibility between the fiber drawing and polymer coating process and the metal plating process. In particular, such processes involve different process speeds and different line travel directions. Drawing, for example, is a vertical process and its line speed is typically more than 10 meter/min. On the other hand, a continuous plating process would be a horizontal process with a typical line speed of less than several meter/min.
So, in the production of metal-coated optical fiber, the optical fiber needs to be temporarily coated during the drawing process and taken into reel. The protective coating is applied because it is difficult to wind a drawn bare fiber into a reel as it is. As one skilled in the art will appreciate, bare fiber without a protective coating is fragile and can easily be broken by touching any hard material. After being reeled in this way, the optical fiber can be paid off from the reel for plating.
Referring now to FIG. 1, a typical plating process of the prior art is illustrated. As will be appreciated, electroplating or electroless plating is a wet process which means the fiber is immersed in liquid solution baths. (The jagged lines in each bath vessel represent the liquid surface.) The metal is coated on the glass fiber as a result of chemical reaction in a liquid. Typically, optical fiber is dipped in several baths in series for cleaning, sensitizing, activation and plating. Each wet process takes one to several minutes to occur in each of the baths. (The process time depends on coating thickness and temperature.) As illustrated by the downward arrows in FIG. 1, short fibers or fiber ends are dipped in a bath. (See, for example, U.S. Pat. No. 5,380,559, incorporated herein by reference in its entirety for all purposes.) Each target fiber moves vertically and is immersed in a bath. After sufficient time, it is retracted from the bath and moved horizontally to the top of the next bath. The plating will be done by soaking in a series of such baths.
Referring now to FIG. 2, conventional metal wire can be coated in a continuous process. As shown, the wire contacts one or more cathode rolls as it proceeds through the process. Additional pulleys may also be provided along the process direction to facilitate movement of the wire. One skilled in the art will appreciate that conventional wire (unlike optical fiber) is robust for handling or bending with tension because it is made of metal rather than glass. (See U.S. Pat. Nos. 5,342,503 and 3,994,786, each of which is incorporated by reference in its entirety for all purposes.)
Optical fiber cannot be coated using the same coating process that would be used with metal wire. If the bare fiber is prepared and enters into wet baths by contacting with pulleys, the pulleys can damage the fiber. In particular, such pulleys are typically made of plastic or metal on the surface, which is hard and can damage the fibers through contact. Even if the pulleys are made of soft material, small dust of hard particles such as silica or metal or any solids may cause damage to the optical fiber's surface due to fiber tension when some such particles exist between fiber and pulley. The bare fiber travels along path line in contact with some pulleys and therefore mechanical damage is caused at some points along the fiber length statistically. So bare fiber is not applicable to metallic continuous plating process to achieve long metal-coated optical fiber. As a result, most application of metal plating to optical fiber is metallization of short ends of optical fiber.
The present invention recognizes the foregoing considerations, and others, of the prior art.